Thin layer mortar blockwork may be constructed with mortar joints of only 2-3 mm, providing that the aircrete or equivalent blocks have been manufactured to fine tolerances and on-site workmanship is good. The special rapid-setting mortar sets typically within 30 minutes and the full bond strength is achieved after only two hours, allowing more courses to be laid each day. In the case of brick and block cavity construction, the inner leaf is built first, providing a weatherproof enclosure as quickly as possible. The outer skin of brickwork can subsequently be built up, using wall ties fixed to the face, either screwed or hammered into the completed blockwork. Bed joints in thin layer mortar blockwork do not co-ordinate with those of the brickwork, so conventional cavity wall ties can only be used if they are slope-tolerant.
Usually inner leaf construction commences with a line of 440 X 215 mm standard height blocks, with normal bedding mortar to compensate for variations in the foundation level, followed by the larger 440 or 620 X 430 mm high blocks, which should weigh less than 20 kg for repeated lifting by one
operative. Heavier blocks require mechanical lifting or two-person handling. Thin-joint mortars, consisting of polymer-modified 1 : 2 cement : sand mix with water-retaining and workability admixtures, are factory pre-mixed and require only the addition of water, preferably mixed in with an electrically-powered plasterer's whisk. The mortar is applied manually with a serrated scoop or through a pumped system to achieve uniformity.
The main advantages of thin-joint systems over traditional 10-mm-joint blockwork are:
• increased productivity allowing storey-height inner leaves to be completed in one day;
• up to 10% improved thermal performance due to reduced thermal bridging by the mortar;
• improved airtightness of the construction;
• the accuracy of the wall allows internal thin-coat sprayed plaster finishes to be used;
• higher quality of construction and less wastage of mortar.
The acoustic properties of thin-joint mortar walls differ slightly from walls constructed with 10-mm-mortar joints. Resistance to low frequency noise is slightly enhanced, whilst resistance to high frequency sound is slightly reduced.
Completed thin-joint blockwork acts as a monolithic slab, which if unrestrained may crack at the weaker points, such as near openings. To avoid this, the block units should be laid dry to avoid shrinkage, and bed-joint reinforcement (1.5 mm thick) should be appropriately positioned. Larger structures require movement joints at 6 m centres.
Certain extruded multi-perforated clay and calcium silicate blocks, available in Europe, are designed for use with thin mortar bed-joints and dry interlocking vertical joints. Whilst this reduces the initial construction time, both sides of the units subsequently require plaster or cement render to minimise heat loss by air leakage.
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